FLAME RETARDANTS

INTRODUCTION Since plastics are synthetic organic materials with carbon and often high hydrogen contents, they are combustible. For various applications in the building, electrical, transportation, mining, and other industries, plastics have to fulfill flame retardancy requirements laid down in mandatory regulations and voluntary specifications. The objectives in flame retarding polymers are to increase ignition resistance and reduce rate of flame spread. Polyolefins are flammable and will burn in air with a hot and clean flame, accompanied by melting and then dripping or flowing of the molten polymer. The flammable nature of polyolefins is due to the long chain saturated hydrocarbon structure which readily fragments at high pyrolysis temperature, yielding highly volatile low molecular weight saturated and unsaturated hydrocarbons that subsequently undergo free radical and oxidation reactions in the pre-ignition zone and flame zone. Ignition Source

The reaction of combustible gases with oxygen is an exothermic reaction, which upon exceeding the endothermic pyrolysis reaction, initiates flame propagation. A flame retardant should inhibit or even suppress the combustion process. Depending on their nature, flame retardants can act physically or chemically. PHYSICAL ACTION By Cooling: The additives cools the substrate to a temperature below the combustion temperature (e.g. ATH)

By Formation of Protective Layer: A solid or gaseous protective layer, which excludes the oxygen necessary for the combustion process (e.g. phosphorus compounds) By Dilution: The inert gases from the additive dilutes the fuel in the solid and gaseous phase (e.g. Aluminum hydroxide) CHEMICAL ACTION Reaction in Gas Phase: The radical mechanism of combustion is interrupted and exothermic reactions are stopped. System cools down (e.g. Halogenated flame retardants) Reaction in Solid Phase: By forming carbonaceous layer on the polymer surface (e.g. phosphorus compounds)

Types of Flame Retardants Flame-Retardants are classified into two categories: additives (mechanically blended with polymeric substrate) and reactives (chemically bound to the polymer). Additive types are used especially for thermoplastics, while reactives are typically used for thermosets. The four main classes of additive type flame retardant for thermoplastics are: Halogenated Compounds Phosphorus Compounds Metallic Oxides Inorganic Fillers

HALOGEN FLAME RETARDANTS The effectiveness of halogen containing flame retardants increases in the order F<Cl<Br<I. Fluorine- and iodine-based flame retardants are not used in practice because neither type interferes with the combustion process at the right point. Fluorine, due to its strong bond with carbon, and iodine is loosely attached to carbon. Bromine compounds are the most effective flame retardant. MODE OF ACTION The hydrogen and hydroxyl free radicals formed by breaking of chemical bonds in the polymer are high in energy and will act as fuel for the combustion process. This radical mechanism takes place in the gas phase and is interrupted by halogenated flameretardants. The exothermic processes are thus stopped, the system cools down, and the supply of flammable gases is reduced and eventually completely suppressed. Thus, the efficiency of the FR system depends not only on the bromine content but also on the molecule to which bromine is attached. The chemistry of the FR system will govern as to when it will start trapping the high-energy free radicals.

RX X* + RH HX + H* HX + OH*

R* + X* R* + HX H2 + X* H2O + X*

The X* radicals formed by series of reactions are low in energy. BROMINE-CONTAINING FLAME RETARDANTS Bromine can be bound aliphatically or aromatically in flame-retardants. The more effective aliphatic types are less stable, while the aromatic types are widely used because of higher thermal stability. CHLORINE-CONTAINING FLAME RETARDANTS Chlorinated flame-retardants are available in chlorinated hydrocarbon form or chlorinated cycloaliphatic form. They are less expensive and offer good light stability, but require a high loading to achieve the required flame retardancy. They are thermally less stable and more corrosive to the equipment when compared to brominated retardants. HALOGEN/ANTIMONY SYNERGISM Antimony trioxide shows no flame-retardant action on its own. However, it shows good synergistic effect with halogen containing compounds. Sb2O3 + HBr 5SbOBr 4 Sb4O5Br2 3Sb3O4Br~ 250C 245 to 280C 410 to 465C 475 to 495C

2SbOBr + H2O Sb4O5Br2 + SbBr3 5 Sb3O4Br + SbBr3 4Sb2O3 + SbBr3

The antimony trihalides and various antimony oxyhalides act as radical interceptors like HCl or HBr. PHOSPHOROUS-CONTAINING FLAME RETARDANTS Phosphorus-based flame retardants can be organic, inorganic, or elemental. They can be active in the vapor phase or in the condensed phase, or sometimes in both phases. Phosphine oxides and phosphate esters are thought to act in the vapor phase through the formation of PO* radicals, which terminates the highly active flame propagating radicals (OH* and H*). The condensed phase mechanism arises as a consequence of

thermal generation of phosphoric acids from the flame retardant, e.g. phosphoric acid or polyphosphoric acid. These acids act as dehydrating agents, altering the thermal degradation of the polymer, and promoting the formation of char. INORGANIC FLAME RETARDANTS ALUMINUM HYDROXIDE Currently this is the most widely used flame retardant. It is low cost and easy to incorporate in plastics. Aluminum hydroxide starts to break down in the temperature range of 180 to 200C. Conversion to aluminum oxide takes place in an endothermic reaction. MAGNESIUM HYDROXIDE Magnesium hydroxide acts in a similar way as aluminum hydroxide, but magnesium hydroxide has a higher decomposition temperature of 300C. ZINC BORATE Boron containing compounds act through the endothermic, stepwise release of water and by the formation of a glassy coating protecting the substrate. FLAMMABILITY TESTS UL-94 VERTICAL BURNING TEST A bar 5 x x 1/8 or 1/16 in thickness is mounted vertically in a draft-free enclosure. The burner is held in place under the sample for 10 seconds and withdrawn and the duration of the flaming is timed. The test is repeated for five specimens. Any dripping which ignites a piece of surgical cotton placed 12 below the bar is noted. Classification: 94 V-0 No specimen shall have flaming combustion for more than 10 seconds after each ignition. No specimen will burn up to the holding clamp. No specimen can drip and ignite the cotton. No specimen will have glowing combustion persisting 30 seconds after the second removal of the test flame. 94 V-1 No specimen shall have flaming combustion for more than 30 seconds after each ignition. No drip

No specimen will burn up to the holding clamp. No specimen will have afterglow of more than 60 seconds. 94 V-2 Same criteria as V-1 except specimens are allowed to drip and ignite the dry surgical cotton below the specimen. LIMITING OXYGEN INDEX (LOI) It is the minimum concentration of oxygen expressed as volume percent in a mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature. UL-181 This test method is used for the fabrication of air duct and air connector systems for use in accordance with the standards of the National Fire Protection Association for installation of air conditioning and ventilating systems. UL-214 This test method is a standard for flame propagation of fabrics and films. Normally the readings are reported as the length of the charred material. A standard specimen of 2 x 10 is cut both in machine- as well as transverse-direction (5 of each) for the small flame test (flame height: 1 ), and 5 x 30 to 84 for large flame test (11 high). The bottom of the specimen is ignited for 12 sec and the flame is removed. The length of the charred material is measured and reported. E-84 This fire-test response standard for the comparative surface burning behavior of building materials is applicable to exposed surfaces such as walls and ceilings. The purpose of this test method is to determine the relative burning behavior of the material by observing the flame spread along the specimen. Flame spread and smoke density data are reported, however there is not necessarily a relationship between these two measurements. MOTOR VEHICLE SAFETY STANDARD 302 This standard specifies burn resistance requirements for materials used in the occupant compartments of motor vehicles. The federal standards requires that a specimen 100 x 356 x thickness (as utilized in the vehicle except where it exceeds 12.7mm, in that situation the specimen is reduced to 12.7 mm, shall not burn, nor transmit a flame front across its surface, at a rate of more than 4 inches/min. Each car manufacturer (Ford, GM, and Chrysler) has its own standards.

101888 100530 101834 101199

LDPE LDPE LLDPE LDPE

60 70 60 52

Not Sure 9-10 10-12 12-14

100 15 13-15 16-18

Non Blooming, high thermal stability 190705 LLDPE 50 12-14 20 Black FR MB, Non Blooming, high thermal stability LR-92743 LLDPE 60 10-12 13-15 Non Blooming, high thermal stability as compared to LR92472 101121 LDPE 2.5 N/A N/A Smoke Suppressant, good UV properties 100532 mLLD 50 Not Sure 100 Non halogenated **NOTE: The V-0 rating of a material also depends on factors such as the molecular structure of the letdown resin (since that constitutes major chunk of the final compound) as well as the additives, which go with them. Higher the molecular weight lower the MFI greater the chances of achieving V-0. Molding grade polyolefins normally have a high tendency to drip because of its high MFI thus making it difficult to achieve V-0 rating. Under such circumstances, filler such as clay should be recommended at 10-14% letdown. The additives, which are basic in nature (such as CaCO3, DE etc.), reduce the efficiency of the halogenated flame-retardants. Higher FR MB letdown should be recommended in that case.

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